The Pajarito Dynamics (PAD) program models dynamic systems with interactive neutronics, thermo- dynamics, and hydrodynamics in one-dimensional spherical, cylin- drical, and planar geometries. The program has been applied to prompt critical excursions in various fissioning systems (solution, metal, LMFBR, etc.) as well as to nonfissioning systems. PAD appli- cations have included: controlled (nondestructive) and damaging reactor transient experiments, hypothetical LMFBR disassembly and recriticality accidents, hypothetical criticality accidents invol- ving the handling and processing of various forms of fissile mate- rials, and nonneutronic systems employing only the thermodynamic and hydrodynamic aspects of the program.

A system is divided into discrete, contiguous regions, or mass points. The neutronics is comprised of one- dimensional Sn transport calculations for determining the spatial power profile and material motion reactivity feedback and analytic models for input and temperature feedback reactivities. Equation-of- state models in the program describe the thermodynamics of solids, liquids, and vapors and also phase transitions. Heat transfer bet- ween two materials at a mass point is considered. The hydrodynamics (acceleration, velocity, position) proceeds in a Lagrangian coordi- nate system. The neutronic-thermodynamic-hydrodynamic cycle is repeated stepwise in space and time until the desired calculational endpoint is reached.

A 16-group, S4, 20 mesh-point model of an LMFBR running for 10,000 iterations (time-steps) an including 50 Sn calculations with a total of 20,000 inner iterations requires about two minutes of CDC7600 time. The longest running sample problem requires approximately 95 CP seconds on the CDC7600.

The internal geometric reactivity change is based on DTF4 (NESC Abstract 209). PAD disassembly calcu- lations for a representative LMFBR model have been compared with those obtained from the FX2-POOL and VENUS2 (NESC Abstract 511) programs.

PAD requires system input and output units and a bulk storage device (e.g., disk or tape) on which four coded files may be defined. Each file should have a 32,000 (octal), 60-bit word capacity. PAD requires approximately 140K (octal) words of storage for execution on a CDC7600.

PAD was developed for the CDC RUN compiler with the rounded arithmetic option. Sample problem 2 terminates abnormally if rounded arithmetic is not used. Memory must be preset to zero prior to execution. Dummy routines are included in the package in place of several LANL com- puting environment routines. The dummy routines and their functions are: XIT, an error interception routine; XITFIX, a routine to pro- vide the application program with the job status, program counter and program status words; GETHPK, a routine to provide the hardware exchange package; DMPPK, which prints the exchange package; DMP, a routine to dump small core memory locations; and GETQ, which pro- vides various job parameters such as the job time limit, to the calling program.